Recurrent miscarriage is defined as 2 or more failed clinical pregnancies (i.e., documented by ultrasound or histopathology).  It affects about 1% of all fertile couples trying to conceive, in comparison with sporadic non-consecutive miscarriages, which occur in about 15% to 20% of all pregnancies.  A miscarriage includes any pregnancy that ends before the age of viability, which currently stands at 24 weeks' gestation. A miscarriage that occurs before 12 weeks' gestation is commonly termed an early or first-trimester miscarriage, and one that occurs between 13 and 24 weeks' gestation is known as a late or second-trimester miscarriage.
Evaluation can start after 2 or 3 consecutive miscarriages, as prevalence of causes is similar in those with 2, 3, or more miscarriages.  Despite a wide range of investigations, no apparent cause is found in more than 50% of cases of recurrent miscarriage.  In spite of this, about 70% of patients with no cause found will achieve a live birth in the subsequent pregnancy depending on the age of the woman and the number of previous miscarriages.  
Definite associations of recurrent miscarriage include chromosomal abnormalities, antiphospholipid syndrome, certain structural uterine abnormalities such as cervical incompetence, and certain thrombophilias. However, a reduction in risk of miscarriage in a subsequent pregnancy following treatment has not been proven unequivocally for most of these conditions. Controversy also surrounds the possible association of other conditions with recurrent miscarriage, including immunological factors, other uterine abnormalities (e.g., bicornuate or septate uterus), and endocrinological factors. There is a need for high-quality and methodologically sound research to guide management of these patients.
Increasing maternal age reduces the chance of a successful live birth. Women aged 20 years with 2 previous miscarriages have a 92% chance of success in the next pregnancy compared with only a 60% chance of success in women aged 45 years. 
Paternal age also plays a part. Frequency of chromosomal anomalies in sperm appears to increase with age. Independent of maternal age, paternal age of more than 40 years carries 1:6 odds of miscarriage compared with paternal age of 25 to 29 years.  Older couples have the worst pregnancy rates and outcome. 
Primigravidas and patients who consistently have successful pregnancies have only about 5% risk of miscarriage, compared with 24% in patients who have previously miscarried.  Other studies similarly show a trend of miscarriage rate increasing with the number of previous miscarriages.   Therefore, the risk of miscarriage is directly related to the outcome of previous pregnancies.
Recurrent miscarriage is a stressful condition, so alongside medical investigations and appropriate treatment, patient education, counselling, and support should be provided.
Dell Medical School
University of Texas
WM declares that she has no competing interests.
Dr Winifred Mak would like to gratefully acknowledge Dr Ai-Wei Tang and Dr Siobhan Quenby, the previous contributors to this topic. AT declares that she has no competing interests. SQ is an author of a reference cited in this topic. SQ declares that she has no competing interests.
Professor of Obstetrics and Gynaecology
Leicester Royal Infirmary
JCK is an author of a reference cited in this topic.
Erasmus Medical Center
FVD declares that he has no competing interests.
Associate Professor of Obstetrics and Gynecology
Washington Hospital Center
VGL declares that she has no competing interests.
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